# To find the modulus of elasticity of a light elastic string

• gnits
In summary, the conversation discusses a problem involving a framework made of four uniform rods connected by an elastic string. The goal is to find the modulus of elasticity of the string. After some discussion and input from other individuals, the person eventually solves the problem using the principle of virtual work, which states that in an equilibrium system, the net work done by external forces is zero. This leads to the equation T = 4w - w/2, which can be solved to find the modulus of elasticity.
gnits
Homework Statement
To find the modulus of elasticity of a light elastic string
Relevant Equations
Moments

Four uniform rods of equal length l and weight w are freely jointed to form a framework ABCD. The joints A and C are connected by a light elastic string of natural length a. The framework is freely suspended from A and takes up the shape of a square. Find the modulus of elasticity of the string.

Formula for modulus of elasticity is M = T * a/x where T is the tension in the string, a the natural length and x the extension.

Here's my diagram:

The book answer is M = 2aw / ( sqrt(2) * l - a)

I have shown from the diagram above that the extension of the string is sqrt(2) * l - a

So what remains is for me to show that the tension in the string is 2w.

I've split the diagram in half and shown the internal reactions in the hinges at B and D and by considering section BC alone and taking moments about C have shown that Y = w/2

But I'm not seeing a way to find T = 2W. I suspect my force labelling may be wrong?

Thanks,
Mitch.

Last edited:

Incidentally, the equation for the modulus of elasticity is incorrect. Just consider the case where the string is in stretched, with a=x.

Thanks for the reply. From my diagram if I take moments of rod AD about D I get:

T1 * l/sqrt(2) - T * l/sqrt(2) - w * (l/2)*(1/sqrt(2)) = 0

Can divide both sides by l/sqrt(2) to give:

T1 - T - w/2 = 0

Resolving vertically for the whole system I have that T1 = 4w and so the equation above becomes:

T = 4w - w/2 leads to T = 7w/2

Not what I had expected. What is my mistake?

Concerning the formula, I am using the following from the textbook I am working with

Thanks again,
Mitch.

gnits said:
From my diagram if I take moments of rod AD about D I get:

T1 * l/sqrt(2) - T * l/sqrt(2) - w * (l/2)*(1/sqrt(2)) = 0
You have not accounted for all of the forces that act on rod AD. For example, at point A, rod AC can exert a force on rod AD.

Also, to maintain symmetry in the problem, I would assume that the supporting force ##T_1## is split such that ##\large \frac{T_1}{2}## acts at point A on rod AB and ##\large \frac{T_1}{2}## acts at point A on rod AD. Likewise, the elastic force ##T## can be assumed to split equally between rods AB and AD at point A and equally between rods BC and CD at point C.

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If you are familiar with the principle of virtual work then you can deduce the elastic tension ##T## fairly quickly without setting up any torque equations or drawing any free-body diagrams for the individual rods.

Thanks very much indeed Chestermiller and TSny for your help. Very much appreciated. It has enabled me complete the question.

gnits said:
Thanks very much indeed Chestermiller and TSny for your help. Very much appreciated. It has enabled me complete the question.
That's good because I'm still struggling with this deceptively simple problem.

I finally solved this, rather inelegantly, by doing force balances on an upper and a lower strut in both the normal and the axial directions to a strut, to get the reaction forces on the ends of the struts. This showed that the vertical component of force exerted by each strut on the right- and left-side connectors is zero. This simplified the vertical force balances on the upper and lower halves of the assembly.

TSny
The principle of virtual work provides a nice way to determine the tension ##T## in the elastic cord.

The principle states that if a system is in equilibrium and you imagine any virtual, small displacement of the system which is consistent with any constraints, then the net work done by the external forces equals zero.

The figure on the left shows the system in equilibrium. The external forces are the support force ##T_1## at A, the elastic string forces ##T## at A and C, and the total weight ##4W## of the system which acts at the center of mass of the system.

We consider a virtual displacement of the system where point A stays fixed while B and D move apart so that the square is distorted into a rhombus. The center of mass will rise by a virtual displacement ##\delta y_{cm}## and point C will rise by ##\delta y_C##.

It is not hard to see that ##\delta y_C = 2 \delta y_{cm}##.

The only external forces that do work during the virtual displacement are the weight and the elastic force at C. So, the principle of virtual work implies

##-(4W) \delta y_{cm} +T \delta y_C = 0##.

This yields ##T = 2W##.

Lnewqban, Chestermiller and berkeman

## 1. What is the modulus of elasticity?

The modulus of elasticity, also known as Young's modulus, is a measure of the stiffness or rigidity of a material. It represents the amount of stress that a material can withstand before it starts to deform.

## 2. How is the modulus of elasticity calculated?

The modulus of elasticity is calculated by dividing the stress applied to a material by the strain it produces. This can be represented by the equation E = σ/ε, where E is the modulus of elasticity, σ is the stress, and ε is the strain.

## 3. Why is it important to determine the modulus of elasticity of a light elastic string?

The modulus of elasticity is an important property to know for any material, as it helps to understand how it will behave under stress. In the case of a light elastic string, knowing its modulus of elasticity can help in designing and using it for various applications such as in sports equipment or musical instruments.

## 4. What factors can affect the modulus of elasticity of a light elastic string?

The modulus of elasticity of a light elastic string can be affected by factors such as its composition, temperature, and the amount of stretching it has undergone. It can also vary depending on the direction of the applied force and the rate at which the force is applied.

## 5. How is the modulus of elasticity of a light elastic string experimentally determined?

To determine the modulus of elasticity of a light elastic string, an experiment can be conducted where the string is subjected to different amounts of stress and the corresponding strain is measured. The data can then be used to calculate the modulus of elasticity using the equation mentioned in the second question.

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